Light valves have been known for over sixty years for modulation of light. As used herein, a light valve may be described as a cell formed of two walls that are spaced apart by a small distance, at least one wall being transparent, the walls having electrodes thereon usually in the form of transparent conductive coatings. The cell contains an activatable material which may be either a liquid suspension of a plastic film in which droplets of liquid suspension are distributed and encapsulated. The liquid suspension (sometimes herein called a liquid light valve suspension) comprises small particles suspended in a liquid suspending medium. In the absence of an applied electrical field, the particles in the liquid suspension exhibit random Brownian movement, and hence a beam of light passing into the cell is reflected, transmitted or absorbed, depending upon the cell structure, the nature and concentration of the particles and the energy content of the light. When an electric field is applied through the light valve suspension in the light valve, the particles become aligned an for many suspensions most of the light can pass through the cell. Light valves have been proposed for many purposes including, e.g., alpha-numeric displays, television displays, windows, sunroofs, sunvisors, mirrors, eyeglasses and the like to control the amount of light passing therethrough. Light valves of the type described herein are also known as "suspended particle devices" or "SPDs".
For many applications, it is preferable for the activatable material to be a plastic film rather than a liquid suspension. For example, in a light valve used as a variable light transmission window a plastic film in which droplets of liquid suspension are distributed is preferable to a liquid suspension alone because hydrostatic pressure effects e.g., bulging, associated with a high column of liquid suspension can be avoided through use of a film, and the risk of possible leakage can also be avoided. Another advantage of using a plastic film is that in a plastic film the particles are generally present only within very small droplets, and hence do not noticeably agglomerate when the film is repeatedly activated with a voltage.
A type of light valve film made by phase separation from a homogeneous solution is disclosed in U.S. Pat. No. 5,409,734. Light valve films made by crosslinking emulsions are disclosed in U.S. Pat. Nos. 5,463,491 and 5,463,492 assigned to the assignee of the present invention. All of those patents and other patents cited herein and references therein are incorporated herein by reference.
For use in set suspensions such as light-polarizing sheets, sometimes called "sheet polarizers", which can be cut up and formed into polarized sunglass lenses or used as filters, light-polarizing particles can be dispersed or distributed throughout a sheet of suitable film-forming material, such as cellulose acetate or polyvinyl alcohol or the like. See e.g., U.S. Pat. Nos. 2,178,996 and 2,041,138. In these set suspensions, however, the particles are immovable.